Multimode ionization cell for gas chromatographic detection

A user-selectable, multimode, beta-ionization cell has been developed for gas chromatographic (GC) detection. The system consists of a modified dc current (nonpulsed) electron capture cell enclosed in a stainless-steel vacuum chamber. A gas mixing manifold connected to the input of the detector enables various reagent gases to be mixed with the GC effluent prior to entering the detector cell. Simply by varying the pressure of the reagent gas inside the detector from atmospheric to as low as 50 mTorr, one of four different modes of operation can be achieved. These include (1) conventional electron capture detection (atmospheric pressure), (2) cross-section ionization electron emission (< 1 Torr), (3) low-pressure argon ionization electron emission (1-10 Torr), and (4) mixed electron capture/electron emission (100-300 Torr). One advantage of this detector is the ability to switch between selective detection (electron capture) and universal detection (argon ionization) by only changing the operating pressure in the cell. Another important feature is that the degree of selectivity is continuously tunable in the mixed electron capture/electron emission mode of operation. This is due to a dramatic pressure dependent competition between argon ionization electron emission and capture of near-thermal electrons by compounds passing through the detector. This selectivity appears to be strongly related to the electron affinity of compounds, with those exhibiting the highest electron affinities capable of capturing electrons at the lowest pressures. This enables classes of compounds, such as polychlorinated biphenyls (PCBs), to be detected in the presence of hydrocarbons. Additionally, compounds such as isomeric polycyclic aromatic hydrocarbons (PAH) can be readily distinguished using this detector.